Endocardial electrodes include an exposed metallic electrode tip at the end of a tubular insulating sheath, the sheath generally being made of silicone rubber, polyurethane or other insulating material which is generally inert to body fluids. The sheath surrounds an electrically conductive electrode for providing electrical continuity between the electrode tip and the other end which usually terminates in a connector that is removably attached to a pulse generator such as a heart pacemaker. This particular type of electrode is known as a unipolar electrode, the pulse generator case being the other electrode. Another type of electrode is known as a bipolar electrode. It includes two electrically conductive electrodes, one connected to the exposed electrode tip and the other to a second exposed electrode generally in the form of an annular ring near the electrode tip.
Originally, electrode tips did not incorporate a mechanical means for preventing their dislodgement from the heart trabeculae. As endocardial electrodes were improved, a plurality of wedge-shaped protrusions formed of a material substantially inert to body fluids was located immediately behind the electrode tip for engaging the trabeculae and providing some mechanical restraint against tip dislodgement. A number of trabeculae-gripping devices have been utilized, including tines formed out of a plastic or metallic material that are retracted during electrode insertion and then extended to engage the trabeculae. In some configurations the wedges were solid, and in others they were conically shaped and hollow. Some configurations replaced the conical shape by a series of elements of the cone which are referred to as "tines". One configuration has tines extending backwardly from an area immediately behind the tip at an acute angle of approximately forty-five degrees. Such a configuration is described in U.S. Pat. No. 3,902,501 to Citron, et al. Tines differ from closed cones principally in that they are generally more flexible and can be made longer, thus tending to be captured better by the trabeculae. Tines in conventional endocardial electrodes bend toward the cylindrical sheath of the lead during lead insertion, thereby reducing its cross-section. Once the ventrical is entered, the tines expand to an acute angle and engage the trabeculae. However, during insertion of the electrode tip into the trabeculae, the tines are further bent down toward the sheath and compressed. Although the tines occasionally capture one or more of the trabeculae, their inherent resiliency to assume the unrestrained acute angle tends to push the tip out of the area of maximum electrical contact with the heart tissue, thereby acting somewhat as a spring and pushing the electrode tip from its furthest penetration. The endocardial electrode provided by the invention tends to minimize this problem by incorporating tines which provide one resistance when the electrode tip is being pushed into the trabeculae or through a vein, and another resistance when the tip is being withdrawn from the trabeculae or through a vein.